In general, a polyimide resin denotes a high heat-resistant resin, which is prepared by solution-polymerizing an aromatic dianhydride(s), and an aromatic diamine(s) or diisocyanate(s) to obtain a polyamic acid derivative, and ring-closure dehydrating and imidizing the polyamic acid derivatives at high temperature to obtain the high heat-resistant resin.
A polyimide film denotes a film prepared in a thin layer from the polyimide resin. Since the polyimide film has an excellent mechanical and thermal dimensional stability and a chemical stability, it is widely used in an electronic/electric material application, space/aviation application, and telecommunication application.
Particularly, a current trend for an electronic device tends to be light-weight, miniaturized, thin and highly-integrated. This trend results in an increase of heat generation per unit volume, followed by problems resulting from such heat loading. Therefore, an effective heat dissipation of such electronic device is important.
In this regard, graphite is exemplified as a means for heat-dissipation used in such electronic device. Graphite has a graphene-stacking structure, wherein the graphene is a 2-dimensional sheet consisting of a plate of carbon atoms arranged as hexagonal lattice, and, has high thermal conductivity and high mechanical strength.
Such graphite is widely used in an energy-storage material such as a second battery, fuel cell and supercapacitor, filter film, chemical detector, transparent electrode, and heat-dissipation material, and the like.
In particular, there is an increasing interest in the graphite sheet prepared by carbonizing the polyimide films obtained from the polyimide resin and graphitizing them.
Particularly, said graphite sheet is prepared by carbonizing and graphitizing steps, involving respective heat-treatment of polyimide films at different temperatures.
In this regard, heat applied during the carbonizing and graphitizing steps provide brittleness to the graphite sheets resulting from the polyimide films. Accordingly, the graphite sheets tend to have relative poor flexibility.
In this regard, the degradation of the flexibility can be solved by adding heat-sublimatable fillers in said polyimide film, so as to sublimate the fillers and form voids within the graphite sheet, during the carbonizing and/or graphitizing steps for preparing the graphite sheet.
However, roughness of the polyimide film surfaces may be reduced depending on factors such as an average particle diameter and contents, of the fillers. As a result, winding properties of the films may be decreased and protruding trails may be formed on the surface of the film. Further, the voids cause heat-transfer paths of the graphite sheets to be longer, resulting in problems such as a degradation of the thermal conductivity and the like.
Accordingly, in regards to the preparation of the graphite sheets, it is very difficult to simultaneously balance the excellent thermal conductivity and other physical properties of the graphite sheets.
Accordingly, there is high technical need for fundamentally solving these problems.